The vast majority of balers does not have its own engine, but is towed by a tractor, and is powered by coupling the power take-off (PTO) shaft of the tractor to an input shaft of the baler. The baler further has a flywheel connected to the input shaft, and a gearbox that drives the various mechanisms in the baler to pick-up the crop from a field, to form the crop into bales and to wrap twine and/or foil around the bales.
A one-way clutch is typically arranged between the PTO shaft and the flywheel to allow the flywheel to rotate faster than the PTO shaft. The baler input shaft can be operatively decoupled from the tractor PTO, e.g. for transportation of the baler from one field to the other.
A large and heavy flywheel is required in a baler, especially in so called “large square balers”, or “high density balers” to overcome peak loads encountered by the baler gearbox, which occurs e.g. when the plunger of the baler compresses the crop material in the bale chamber when forming the bales. By using a flywheel with a high inertia and running at a high speed (e.g. 1000 rpm), peak energy for the peak loads can be delivered by the flywheel, which slightly slows down during each compression, and which is accelerated again by the tractor PTO between two compressions. However, the high inertia of the flywheel creates a problem when starting the baler. The PTO shaft when driven by the tractor engine at the speed at which it needs to turn to drive the baler during normal operation, may not be able to provide the high torque needed to bring the flywheel up to its steady speed (e.g. 1000 rpm). As a result, when the baler is hitched to the tractor and its flywheel is connected to the tractor PTO shaft, the tractor engine may stall or a safety mechanism may declutch the PTO from the tractor engine on account of the high load on the PTO shaft when the baler is started.
A solution to this problem proposed in EP1974601 is to use a hydraulic motor to supplement the torque provided by the PTO shaft when starting the baler flywheel. This solution is not entirely satisfactory because it requires a powerful hydraulic motor, with a power comparable to that of the tractor PTO.
Another solution to this problem is disclosed in WO2011060995, where a tractor with a change speed gearbox is described, which allows the baler to start-up in two phases. In the first phase the flywheel is accelerated to a first speed using a first transmission ratio of the change speed gearbox, and in the second phase the flywheel is accelerated to a second speed using a second transmission ratio of the change speed gearbox. A disadvantage of this technique is that it requires a tractor with a change speed gearbox.